fix selects: use child scope inside the directive

Before, if we wanted to use a scope method inside the
"lx-select-selected" or "lx-select-choices", we couldn't because the
directive scope was isolated.
By using a child scope, we keep access to all of the data from the
parent scope. Making this, we now use "ng-model" to manage the model as
the two-ways binding is only doable in an isolated scope.

BROKEN:
selects now use "ng-model" instead of "model"
fix tooltip: remove element on scope destroy

Sometimes the tooltip was still visible in the screen even after a page
change. By deleting the tooltip's element from the DOM when the scope is
destroyed, we ensure that the tooltip don't stay visible.
fix dropdown: remove element on scope destroy

Sometimes the dropdown was still visible in the screen even after a page
change. By deleting the dropdown's element from the DOM when the scope
is destroyed, we ensure that the dropdown don't stay visible.